In recent years, mirror displays in which a half mirror plate that includes a half mirror layer is arranged on the front surface of a display device so that the mirror display can function as a mirror when no images are being displayed have been developed for use in applications such as digital signage (see Patent Document 1, for example). These mirror displays can be used as mirrors in addition to being used for the original intended purpose as displays. In other words, in these types of mirror displays, while the display device is emitting display light, images are displayed using that display light in the region from which the display device emits the display light. Meanwhile, while the display device is not emitting display light, the region from which the display device is not emitting display light functions as a mirror due to the reflection of external light off of that region.
Examples of well-known materials suitable for use as the half mirror layer include deposited metal films, dielectric multilayer films, and the like. Furthermore, mirror displays in which a reflective polarizer is used as the half mirror layer in order to make it possible to improve the visibility of the image in the bright region of the mirror display while in display mode have also been developed (see Patent Document 2, for example). Well-known examples of such reflective polarizers include multilayer reflective polarizers, cholesteric liquid crystals used in combination with a quarter-wave plate, wire-grid reflective polarizers, and the like. One example of such a multilayer reflective polarizer is a Sumitomo 3M reflective polarizer (product name: DBEF). One example of a reflective polarizer in which cholesteric liquid crystals are used in combination with a quarter-wave plate is a Nitto Denko Corporation reflective polarizer (product name: NIPOCS).
In mirror display applications, it is often difficult to achieve sufficient visibility when the mirror display is used as a mirror in dark environments because the mirror display must reflect external light in order to function as a mirror. Moreover, even in mirror displays in which a reflective polarizer is used as the half mirror layer, it is difficult to achieve a reflectance equal to or greater than that of a normal mirror when the mirror display is used in mirror mode simply due to the principle by which mirror mode works. Therefore, there is strong demand for improved visibility of reflected images in mirror mode in these types of mirror displays, with poor visibility of reflected images in dark environments being a problem of particular significance. To solve this problem, mirror displays that include an auxiliary illumination unit in order to illuminate reflected objects (such as an onlooker's face) when in mirror mode have been developed (see Patent Documents 3 to 5, for example).